1. Field of the Invention
The invention involves a plating bath, and a corresponding method, for electrolessly depositing a metal, e.g., copper, onto a substrate, such as an electronic packaging substrate, as well as the resulting, metallized substrate.
2. Description of the Related Art
The use of an electroless plating bath for electrolessly depositing a metal, e.g., copper, onto a substrate, is now a common practice in the manufacture of a variety of electronic packaging substrates, such as printed circuit boards. Such an electroless plating bath conventionally includes: (1) water; (2) a soluble compound containing the metal to be deposited onto the substrate of interest; (3) a complexing agent for the corresponding metal ions, which serves to prevent chemical reduction of the metal ions in solution while permitting selective chemical reduction on a surface of the substrate; (4) a chemical reducing agent for the metal ions; (5) a buffer for controlling pH; and (6) small amounts of additives, such as bath stabilizers and surfactants.
The electroless plating baths used to deposit, for example, copper onto printed circuit board substrates conventionally include copper sulfate as the source of copper, ethylenediaminetetraacetic acid (EDTA) as the complexing agent and formaldehyde as the reducing agent. Obviously, the use of formaldehyde as a reducing agent in such baths is undesirable because it poses health and safety problems for human beings. Moreover, such baths can only operate at pH 11 or greater. But, this is considered undesirable because certain substrates, such as polyimide substrates, cannot withstand such high pHs, over the times and temperatures needed to achieve copper plating.
One attempt at overcoming the above-described drawbacks associated with conventional copper plating baths has involved the use of amino boranes, e.g. , dimethylaminoborane, as reducing agents. While these reducing agents do not pose the health and safety problems that formaldehyde poses, their relatively high cost has limited their use to small volume, high end electronic packaging substrate products.
Yet another attempt at overcoming the above-described drawbacks has involved the use of hypophosphite ions (introduced into a copper plating bath as, for example, sodium hypophosphite) as the reducing agent. While hypophosphite is relatively innocuous, it has been found that when used as a reducing agent (in the absence of nickel or cobalt ions, discussed below), the corresponding deposition of copper stops after a very short period of time, with the thickness of the deposited copper being no more than about 1 micrometer. That is, while such a bath is initially autocatalytic in relation to the reduction of copper ions at a substrate surface, it quickly becomes non-autocatalytic. It is believed that this behavior is due to the incorporation of phosphorus (from the hypophosphite) into the substrate surface, which poisons the chemical reduction reaction at the substrate surface.
Significantly, as described in U.S. Pat. No. 4,265,943, issued to Goldstein et al on May 5, 1981, which is hereby incorporated by reference, it has been found that the introduction of nickel ions or cobalt ions into an electroless copper plating bath using a hypophosphite reducing agent serves to overcome the above-described problem. That is, the presence of nickel ions or cobalt ions serves to convert the above-described non-autocatalytic copper-reduction reaction into one which is autocatalytic, resulting in continuous deposition of copper. However, if, for example, nickel ions are used, then it has been found by the present inventors that the resulting deposited copper invariably contains at least 3.63 atomic percent incorporated nickel, while if cobalt ions are used, then it has also been found by the present inventors that the resulting deposited copper invariably contains even more incorporated cobalt. In either event, such relatively large amounts of incorporated nickel or cobalt are unacceptable for many applications.
Thus, those engaged in the development of electroless metal plating baths have sought, thus far without success, techniques for converting an otherwise non-autocatalytic metal-reduction reaction using hypophosphite into an autocatalytic reaction, but which result in less than, and preferably significantly less than, 3.63 atomic percent unwanted metal being incorporated into the desired, deposited metal.